The invention relates to a process for continuously casting unkilled or semi-killed steel with a carbon content of about 0.02 to 0.20%, wherein the steel is cast from a tundish through a casting tube which is immersed in a water-cooled mould, and the solidifying strand is extracted from the mould and further cooled.
There are various purposes for which unkilled steel is the most suited and cannot be replaced by killed steel. Such fields of application are, for instance, zinc coating, enamelling and autogenous welding. But even when a completely faultless surface is needed, unkilled steel is preferred.
Hitherto it has not been possible to cast unkilled steel to qualitatively faultless strands on a large scale. The intensive CO-formation at the front of solidication which is necessary for obtaining an edge zone free from blow-holes, leads to a boiling-over of the steel in the mould. Thus, so far it has not been possible to utilize the great economic advantage of continuous casting for unkilled steel.
Semi-killed steel, too, for qualitative reasons cannot be continuously cast, because here too, it has not been possible to obtain an edge zone free from blow-holes at contents of deoxidation products that are as low as possible. The gas bubbles formed during casting are enclosed by the solidifying strand skin and some of them reach the surface of the cast product. The bubbles rupture during rolling, the cavities oxidize and irreparable faults form at the surface of the rolling stock. Only when one succeeds in obtaining a surface that is free from blow-holes, i.e. when the distance of the gas bubbles from the surface is at least 5 mm, can a faultless surface of the rolling stock be achieved.
In the prior art a number of suggestions have been made for continuously casting unkilled steel. Thus, according to U.S. Pat. No. 3,189,956, prior to casting the steel is first deoxidized and then subsequently oxidized by blowing oxygen onto the freely falling casting stream in order to achieve a controlled boiling. However the certainty of yielding a certain product [ %C] .times. [ %O] in the desired range of 0.0040 to 0.0045% is very low with this process. Also, there is the danger of over-boiling and the inclusion content of such steels is increased due to reoxidation. The freely falling casting stream furthermore increases the danger of break-throughs and causes a poor surface.
USSR Patent No. 1,346,029 indicates that strands of unkilled steel can be produced by introducing inert gas into the tundish and into the mould, the gas nozzles being immersed in the melt in the mould. With this process, however, it is not possible to avoid the precipitation of gas bubbles in the edge zone of the cast product, because it is not possible to supply an amount of inert gas sufficient to flush out the carbon monoxide bubbles without an over-boiling of the melt. Furthermore, in this process casting disturbances may occur due to the formation of big bubbles.